Some of the storage capacity of any storage device, including of your iPhone, iPad, iPod, and Mac, is used by its formatting. Other factors, such as the operating system installed on the device, use additional storage space.

iOS 10 and earlier, Mac OS X Leopard and earlier, Microsoft Windows, and watchOS use the binary system (base 2), which calculates 1GB as 1,073,741,824 bytes. This difference between the decimal and binary systems of measurement is why the reported storage capacity differs from the storage capacity on the product packaging or specifications.